Add support for "Github Flavored Markdown Task Lists"

[Hasan-75]

This resolves issue #404 (Add support for "Github Flavored Markdown Task Lists")

Example:
HTML:

<ul>
  <li><input type="checkbox" checked> Complete the project report</li>
  <li><input type="checkbox"> Submit the report</li>
  <li>This is a regular list item</li>
</ul>

Converted Markdown:

- [x] Complete the project report
- [ ] Submit the report
- This is a regular list item

Markdown Preview:

• Complete the project report
• Submit the report
• This is a regular list item

[Hasan-75]

@mysticmind , I've opened a PR for this issue (#405).
Please take a look when you have a chance. Thanks!

[mysticmind]

Acknowledge your PR, will take a look and pull it in.